The present invention relates to gelled fluids and methods for using liquefied petroleum gas in subterranean operations. More particularly, the present invention relates to servicing fluids that comprise gelled liquefied petroleum gas or servicing fluids that comprise a conventional gelled hydrocarbon fluid with liquefied petroleum gas and methods of using such servicing fluids in subterranean formations.
Servicing fluids are used in a variety of operations and treatments performed in oil and gas wells. Such operations and treatments include, but are not limited to, production stimulation operations, such as fracturing, and well completion operations, such as gravel packing.
An example of a production stimulation operation using a servicing fluid having particles suspended therein is hydraulic fracturing. That is, a type of servicing fluid, referred to in the art as a fracturing fluid, is pumped through a well bore into a subterranean zone to be stimulated at a rate and pressure such that fractures are formed or enhanced in a desired subterranean zone. The fracturing fluid is generally a gel, emulsion, or foam that may comprise a particulate material often referred to as proppant. When used, proppant is deposited in the fracture and functions, inter alia, to hold the fracture open while maintaining conductive channels through which such produced fluids can flow upon completion of the fracturing treatment and release of the attendant hydraulic pressure.
An example of a well completion operation using a servicing fluid having particles suspended therein is gravel packing. Gravel packing treatments are used, inter alia, to reduce the migration of unconsolidated formation particulates into the well bore. In gravel packing operations, particulates, referred to in the art as gravel, are carried to a well bore in a subterranean producing zone by a servicing fluid known as a carrier fluid. That is, the particulates are suspended in a carrier fluid, which may be viscosified, and the carrier fluid is pumped into a well bore in which the gravel pack is to be placed. As the particulates are placed in the zone, the carrier fluid leaks off into the subterranean zone and/or is returned to the surface. The resultant gravel pack acts as a filter to separate formation solids from produced fluids while permitting the produced fluids to flow into and through the well bore. While screenless gravel packing operations are becoming more common, traditional gravel pack operations involve placing a gravel pack screen in the well bore and packing the surrounding annulus between the screen and the well bore with gravel sized to prevent the passage of formation particulates through the pack with produced fluids, wherein the well bore may be oriented from vertical to horizontal and extend from hundreds of feet to thousands of feet. When installing the gravel pack, the gravel is carried to the formation in the form of a slurry by mixing the gravel with a viscosified carrier fluid. Such gravel packs may be used to stabilize a formation while causing minimal impairment to well productivity. The gravel, inter alia, acts to prevent the particulates from occluding the screen or migrating with the produced fluids, and the screen, inter alia, acts to prevent the gravel from entering the well bore.
In some situations the processes of hydraulic fracturing and gravel packing are combined into a single treatment to provide a stimulated production and an annular gravel pack to prevent formation sand production. Such treatments are often referred to as “frac pack” operations. In some cases the treatments are completed with a gravel pack screen assembly in place with the hydraulic fracturing treatment being pumped through the annular space between the casing and screen. In this situation the hydraulic fracturing treatment ends in a screen out condition creating an annular gravel pack between the screen and casing. This allows both the hydraulic fracturing treatment and gravel pack to be placed in a single operation. In other cases the fracturing treatment may be performed prior to installing the screen and placing a gravel pack.
In carrying out hydraulic fracturing, frac packing, and gravel packing, fluid recovery oftentimes is critical. Foamed fluids have been developed in part to provide enhanced fluid recovery through energization by a compressed gas phase. They also reduce the total amount of liquid used, typically by a factor of about four. Such foamed fluids have included various surfactants, known as foaming and foam stabilizing agents, for facilitating the foaming and stabilization of the foam produced when a gas is mixed with the servicing fluid. Thus, foamed fluids may be thought of as media in which a relatively large volume of gas is dispersed in a relatively small volume of liquid, usually with the aid of a surfactant that reduces the surface tension of the fluids. The most commonly used gases for foamed fracture fluids are nitrogen, carbon dioxide, and combinations of the two. Foamed servicing fluids may be preferred over conventional servicing fluids because they generally provide superior fluid recovery as well as excellent fluid loss control without forming a substantial filter cake. Enhanced fluid recovery is provided by the expansion of the gas in the foam when the pressure is released after the stimulation and/or treatment. This promotes flow of residual servicing fluid liquid back into the well, thus aiding in cleanup of the servicing fluid once the subterranean operation is complete.
The use of conventional water-based servicing fluids in subterranean operations may present disadvantages. For instance, the high capillary pressures associated with the use of an aqueous system may restrict the flow of produced gaseous hydrocarbons such as methane. Capillary pressures of several thousand psi may result in low permeability formations, wherein the high pressure differential needed to initiate gas flow may result in extended fluid recovery times, or permanent loss of effective fracture half length. Furthermore, the use of water in under-saturated reservoirs also may reduce permeability and associated gas flow through a permanent increase in the water saturation of the reservoir.
The use of a carbon dioxide miscible hydrocarbon servicing fluid may overcome these limitations through achievement of a miscible drive mechanism where produced methane is used to displace the hydrocarbon fracturing fluid from the formation. To facilitate this process, more volatile hydrocarbon blends may be used in place of traditional hydrocarbon servicing fluids such as diesel fuel. For example, carbon dioxide may be added to the hydrocarbon-based servicing fluids, inter alia, to increase the efficiency by which methane can displace it and provide increased energy for fluid recovery and thus its rate of recovery from the subterranean formation. However, increasing concentrations of dissolved carbon dioxide in the liquid hydrocarbon make it progressively more difficult to gel with phosphate ester and alkylphosphonic acid ester gel systems. As a result there is a limit to the concentration of carbon dioxide that may be present in such servicing fluids. For instance, if too high a concentration of carbon dioxide is present, the servicing fluid may not have a viscosity sufficient to carry the needed quantity of particulates to a desired location within a well bore, to adequately control fluid leak off, and to generate the desired fracture geometry.
Moreover, as a fracture or a gravel pack is created, a portion of the liquid contained in the servicing fluid may leak off into the formation and/or may create a filter cake comprising deposited viscosifier on the walls of the fracture, well bore, or the formation. In addition, conventional water-based servicing fluids may comprise polysaccharide-based polymers, which may serve as a food source for bacteria. Therefore, when deposited in the subterranean formation, such polysaccharide-based polymers may produce a bio-mass that may reduce formation permeability. While formation of a filter cake during pumping may be desirable to help control fluid leak off, it is not desirable for the filter cake to be permanent since it may restrict subsequent gas and liquid flow.